1. Field of the Invention
This invention relates to an image scanning method for a scanner. More particularly, this invention relates to an image scanning method that can improve the image scanning rate by determining the period of the driving signal and the number of rotating steps of the driving motor, and calculating the period of triggering signal for the light-sensitive devices according to a predetermined resolution.
2. Description of Related Art
In an early scanner design, the period of the triggering signal for a light-sensitive device is conventionally used as the timing pulse because the minimum period of triggering signal for the light-sensitive device is limited by the exposure time and the time needed for storing a pixel. Hence the period of the triggering signal for the driving motor is an integral multiple of the period of the triggering signal for the light-sensitive device. As a result, the scanning rate is limited as well.
In FIG. 1A, a charge coupled device (CCD) module 10 includes three CCD sensors, which are respectively used to detect and receive the three primary colors of light. The colors of light are red, green, and blue owing to different wavelengths. The sensors include a red light sensor 100, a green light sensor 110 and a blue light sensor 120. The red, green, and blue light signals received by the light sensors are then combined to duplicate the original image. It is also practical to employ a single light sensor to receive and display a monochromic image. Conventionally, a light-sensitive device receives a signal of exposition generated corresponding to an image illuminated by a light source. Then the light-sensitive device sends a triggering signal 15 to activate the red light sensor 100, the green light sensor 110 and the blue light sensor 120 simultaneously for extracting the image signals of red, green, and blue lights respectively. Normally, the signal lines 105, 115, and 125 are shorted together at one end so that the triggering signal 15 can be fed into the red light sensor 100, green light sensor 110 and the blue light sensor 120 simultaneously. These three sensors 110, 110, and 120 receive the same triggering signal 15 because the signal lines 105, 115, and 125 are connected to each other.
The signal lines 102, 112, and 122 are used to feed received image signals of the three primary colors into the converting module 130. The converting module 130 includes a number of independent analogue/digital converters and a multiplexer 140. The red image signal, green image signal and blue image signal are converted into a red digital signal, a green digital signal, and a blue digital signal by analogue/digital converters 104, 114, and 124 respectively. Then the converted red, green, and blue digital signals are fed into the multiplexer 140 through signal lines 108, 118, and 128. The multiplexer 140 picks one signal from the group consisting of red, green, and blue digital signals according to the actual application, and stores it in storage device 150 through signal line 142.
The multiplexer 140 doesn't switch between signal lines 108, 118, and 128 during the time between two triggering signals 15 in a conventional method, that is, only one corresponding converted digital signal is stored in the storage device 150 within a triggering period. In another conventional method, the multiplexer 140 is switched three times within a triggering period and a pixel period to extract three converted digital signals and store them into the storage device 150.
Generally, a CCD module 10 driven by a motor 4 is used as an example for describing a conventional scanning device. Another functional design employs a motor to drive the document scanner but not the CCD module. A user can predetermine a scanning resolution and start the scanning job according to the predetermined scanning resolution. Normally, the higher the scanning resolution, the more digital data from the three primary colors is needed, which requires a longer scanning time as well, so the processing rate of a CCD module 10 is closely related to the predetermined resolution, As a result, the rotating speed of the motor 4 is determined according to the predetermined resolution. Since the triggering signal of a light-sensitive device is used as a basic timing pulse in a conventional method, the triggering signal 15 is fed into a driver 2 as a basic driving pulse for driving the driving motor 4 to drive the CCD module 10 to scan images. The relationship between the triggering signal 15 and the driving signal is shown in FIG. 1B.
Referring to FIG. 1B, the rotation of the motor 4 is controlled by the driving signal, wherein the driving signal is related to the predetermined resolution of the scanner and the triggering signal 15. For example, for a predetermined resolution of 300 dots per inch (dpi), a corresponding triggering signal 15 has pulses 160 to 166, and the driving signal has pulses 170, 173, and 176. In other words, every three times the CCD module 10 is triggered, the motor 4 rotates one step. Since the CCD module 10 is triggered three times during the time between the rotating steps of the motor 4, and the multiplexer 140 is not switched within every triggering period, three sets of digital data are stored by using the circuit shown in FIG. 1A. The three sets of digital data correspond to the three primary colors, and each one of the digital data is stored within a triggering period. On the other hand, the multiplexer 140 can be switched three times within one triggering period when this conventional method is applied to extract the data of a pixel, that is, all digital data from the three primary colors can be stored within one triggering period. In order to keep the motor 4 rotating at a constant speed and save the space in the buffer memory, which has a limited capacity, there is no data stored within the other two triggering periods.
The following examples are used for describing a different scanning process according to different predetermined resolutions, wherein the multiplexer 140 is not switched within one triggering period.
In the case of a predetermined resolution of 200 dpi, a corresponding driving signal has pulses 180-186 (180, 182, 184, 186) to the triggering signal 15, that is, the motor 4 rotates one step after every two times the CCD module 10 is triggered as shown in FIG. 1B. By utilizing the circuit shown in FIG. 1A, there are two sets of digital data stored within the time needed for motor 4 to rotate one step.
In the case of a predetermined resolution of 100 dpi and 75 dpi, a corresponding driving signal has pulses 190-196 (100, 191, 192, 193, 194, 195, 196) to the triggering signal 15. That is to say, the motor 4 rotates one step after every time the CCD module 10 is triggered, as shown in FIG. 1B. By utilizing the circuit shown in FIG. 1A, there is one set of digital data stored within the time needed for motor 4 to rotate one step.
According to the foregoing, the triggering signal 15 has pulses 160-166 for triggering CCD module 10. Every time the CCD module 10 is triggered, there is one set of digital data selected from the digital data from the three primary colors stored. Since the digital data of the three primary colors are needed in combination in order to duplicate an image, the amount of required digital data is directly related to the predetermined resolution. The higher the resolution, the more color data are required. Generally, the data needed for displaying an image at a resolution of 300 dpi is about three times the data needed for displaying an image at a resolution of 100 dpi. Since the method for extracting colors is not related to the invention, there is no further discussion about it.
If there is only one set of data from which a color can be extracted within one triggering period, the motor 4 has to wait for at least three triggering periods for the CCD module 10 to extract three sets of digital data for the three primary colors at one position before the motor can move to the next position. Furthermore, even though the CCD module 10 only extracts one set of digital data for a monochrome image, the motor 10 still has to wait for one triggering period before it can move the CCD module 10 to the next position for extracting the next set of color data. As a result, the motor speed is limited by the frequency of the triggering signal 15.
Even though the CCD module 10 is able to extract and store three sets of digital color data within one triggering period, that is, the multiplexer 140 is switched three times within one triggering period, but the triggering signal is still used as a basic driving pulse for the motor 4, the rotation speed of motor 4 is still limited by the frequency of the triggering signal 15. Furthermore, since the other two triggering periods are not used for extracting or storing data, it wastes the system's resources.
According to the foregoing, a conventional image scanning method for a scanner has at least the following drawbacks:
1. The CCD module can only store one set of digital data for the three primary colors within one triggering period, which makes the scanning rate low and limited. PA1 2. Because the triggering signal is used as a basic driving pulse, the frequency of the driving signal cannot exceed the frequency of the triggering signal, which limits the scanning rate. PA1 T.sub.G =T.sub.M * N, or PA1 T.sub.M =T.sub.G /N.